As in previous studies on this project, the magnitude of the immune response of mice to the Thy-1 antigens will be measured by the number of cells producing antibodies lytic for target cells carrying the appropriate antigen. The studies will concentrate on the recently demonstrated phenomenon of the effect of the responder H-2 heterozygosity on the anti-Thy-1 response. Specifically, it will be tested whether such an effect is due to complementation of Ir genes in the responder or due to recognition by the H-2 heterozygous responder, the carrier-like molecule expressed by H-2 homozygous donor. To carry out these studies, production of several new congenic strains will be continued and initiated. Furthermore, a systematic screening of various F1 hybrids will be undertaken in search for other cases in which H-2 heterozygosity of the responder results in a good response to Thy-1 antigen presented by donors Thy-1 congenic with one of the parental strains.